External loop airlift photobioreactor for cultivation of Chaetoceros Calcitrans

Abstract

The study of the cultivation of Chaetoceros calcitrans was performed using the 17 L external loop airlift photobioreactor (ELAP) with the ratio between downcomer and riser cross sectional area (A[subscript d]:A[subscript r]) of 3.717. It was demonstrated that lowering the location of the connection tube from 120 cm (in UTCT-ELAP) to 100 cm (in LTCT-ELAP) helped improve the growth performance. This could be due to the improved gas-liquid mass transfer rate which allowed more CO₂ to dissolve into the medium as a carbon source. Inclining the connection tube (in ITCT-ELAP) could facilitate the release of bubbles from the system, however, the inherited higher liquid velocity in such system lowered the gas-liquid mass transfer, and no improvement in the growth was observed when compared with the performance of UTCT-ELAP. Optimal aeration superficial velocity (u[subscript sg]) was found to agree with the literature at 3 cm s⁻¹ where the maximum cell density and specific growth rate from the semi-batch operation of LTCT-ELAP were approx. 11.9x10⁶ cell mL⁻¹ and 0.0846 h⁻¹, respectively. This was because at low [subscript sg] (< 3 cm s⁻¹), the system was operated at low gas-liquid mass transfer which inherently could not allow high growth rate. On the other hand, high usg (> 3 cm s⁻¹) caused too much cell damage and lowered the growth performance. only a slight decrease in maximum cell concentration was observed at u[subscript sg] of 4 cm s⁻¹ as this rather high [subscript sg] could see small bubbles into the downcomer and decreasing the light intensity. When compared with the internal loop airlift photobioreactor (ILAP), ELAP clearly provided a better growth performance due to a lower light shading effect. Owing to the fact that the cultivation in ELAP helped reduce the number of bubbles in the downcomer, this lessened the bubble shading effect which was the growth limiting factor in ILAP, and promoting a better light penetration to cells. Lastly, the pre-evaluation of the effect of CO₂ for the cultivation of C. calcitrans in the LTCT-ELAP demonstrated that, although cells of C. calcitrans could grow in the system with as much as 5% CO₂, the growth was not as good as when the cells were cultivated in normal air (without CO₂). This could be attributed to the high acidity level imposed by CO₂